Voltage-current converting circuit

ABSTRACT

A voltage-current converting circuit comprises a plurality of transistors which are applied at the bases with a multiphase voltage, Δ-connection resistor connected between the emitters of the transistors, and current sources respectively connected to the emitters of the transistors.

BACKGROUND OF THE INVENTION

The present invention relates to a voltage-current converting circuitfor converting an input voltage into a corresponding output current.

With a prior voltage-current converting circuit only a two-phase inputvoltage could be converted to a corresponding current. There has been astrong demand to convert a three-or-more-phase input voltage into acorresponding current.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide avoltage-current converting circuit for converting a multiphase voltagewith three or more phases into a corresponding multiphase current.

The above object is achieved by a voltage-current converting circuitcomprising at least three transistors whose bases are applied with amultiphase voltage and whose collectors are connected to the outputterminals, at least three resistors connected between the emitters ofthe transistors, and at least three current sources respectivelyconnected to the emitters of the transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an embodiment of a voltage-currentconverting circuit according to the invention;

FIG. 2 is a circuit diagram of a second embodiment of the presentinvention;

FIG. 3 is a circuit diagram of a prior art two-phase voltage-currentconverting circuit similar to the second embodiment;

FIGS. 4 and 5 show circuit diagrams of third and fourth embodimentsrespectively corresponding to the FIGS. 2 and 1 embodiments which aremodified into an N-phase voltage-current converting circuit;

FIGS. 6 and 7 are circuit diagrams of fifth and sixth embodiments whichare the FIGS. 1 and 2 embodiments but improved in accuracy;

FIGS. 8 and 9 are circuit diagrams of seventh and eighth embodiments,respectively;

FIG. 10 is a circuit diagram of a circuit used for analyzing theoperating condition of the eighth embodiment;

FIG. 11 is a graph illustrating the operating condition of the eighthembodiment;

FIGS. 12A to 12C show waveforms of input voltages used when the eighthembodiment is subjected to a computer simulation;

FIGS. 13A to 13C are waveforms of output currents with theoreticalvalues;

FIGS. 14A to 14C show waveforms of the output currents with simulatedvalues; and

FIG. 15 is a circuit diagram of a ninth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of a voltage-current converting circuit according to thepresent invention will be described referring to the accompanyingdrawings. FIG. 1 is a circuit diagram of a first embodiment of thepresent invention, which is designed for the conversion of a three-phasevoltage. The input voltages V1 to V3 of the three phases arerespectively applied to the bases of transistors Q1 to Q3. Throughoutthe specification, the transistors used are all of NPN type. Thecollectors of the transistors Q1 to Q3 serve as current output terminalsO1 to O3, respectively. These output terminals pull output currents I1to I3, as shown. The emitters of the transistors Q1 to Q3 are connectedto a negative power source -V through current sources I01 to I03.Δ-connection resistors R12, R23 and R31 are connected to the emitters ofthe transistors Q1 to Q3. The resistor R12 is connected between theemitters of the transistors Q1 and Q2; the resistor R23 between theemitters of the transistors Q2 and Q3; and the resistor R31 between thetransistors Q3 and Q1.

The operation of the present embodiment will be given. It is assumedthat the transistors Q1 to Q3 operate in an active region, and that thecommon emitter current amplification factor of each transistor issatisfactorily large. The collector current (output current) I1 of thetransistor Q1 is the sum of the currents flowing through the currentsource I01 and the resistors R12 and R31. A current flowing through theresistor R12 from the transistor Q1 to the transistor Q2 is given as aquotient when a voltage between the emitters of the transistors Q1 andQ2 is divided by the resistance of the resistor R12, that is, ##EQU1##

Currents flowing through the resistors R23 and R31 are also obtained ina similar way. The collector currents I1 to I3 of the transistors Q1 toQ3 are given ##EQU2## where ΔVBE(IJ) represents a difference between thebase-emitter voltage VBE(QI) of the I-th transistor QI and thebase-emitter voltage VBE(QJ) of the J-th transistor QJ, that is,VBE(QI)-VBE(QJ). Assuming that the base-emitter voltages VBE of thetransistors Q1 to Q3 are equal to each other, and the resistances of theresistors R12, R23 and R31 are all R, the equations (1-1) to (1-3) aresimplified into ##EQU3##

In the equations (2-1) to (2-3), if I01=I02=I03, differences between twocurrents are obtained as follows: ##EQU4##

As seen from the above equations, the difference I1-I2 between theoutput (collector) currents of the transistors Q1 and Q2 is proportionalto a difference V1-V2 between the input (base) voltages of thetransistors Q1 and Q2. Similarly, the current I2-I3, is proportional tothe voltage difference V2-V3, and the current I3-I1 to the voltagedifference V3-V1. Here, a proportional coefficient (mutual conductance)is 3/R. As described above, the present embodiment operates asdifferential type three-phase voltage-current converting circuit.

The description that the base-emitter voltages VBE of the transistorsare equal to one another is true only under the condition as givenbelow. Generally, a relationship between the collector current IC of thetransistor and the base-emitter voltage VBE is approximately describedby a diode formula expressed VBE=Vt·ln(IC/IS). In the formula, Vt is athermal voltage, IS is a reverse saturation current. When using thisformula, differences ΔVBE between the base-emitter voltages of the twotransistors are given ##EQU5##

It is assumed here that the converting circuit is a small signal circuit(V1, V2, V3<δ), and that differential input signals are small, V1-V2≃0,V2-V3≃0, and V3-V1≃0. On this assumption, from the equations (1-1) to(1-3), we have I1=I2=I3=I0 (=I01=I02=I03). When this relation holds,ΔVBE=0 is derived from the equations (4-1) to (4-3), and hence thebase-emitter voltages VBE of the transistors are all equal to eachother. This indicates that the equations (3-1) to (3-3) hold when theconverting circuit is used as a small signal circuit. When the inputvoltage is large and the differential input is not regarded as zero, theequations (3-1) to (3-3) fail to hold. Under this condition, the ΔVBEappears as an error in the output differential current. The compensationof the error will be given later.

Another embodiment of the present invention will be described. Thereference numerals used in the first embodiment will be used forcorresponding portions in the other embodiments. While the firstembodiment uses a Δ-connection resistor arrangement, a second embodimentuses a Y-connection resistor arrangement. In the second embodiment shownin FIG. 2, resistors R1, R2 and R3 are connected at one end to theemitters of the transistors Q1 to Q3. The other ends of the resistorsare connected together.

The operation of the second embodiment will now be described. When avoltage at the common connection point of the resistors R1 to R3 is VS,the sum of the current flowing into the common connection point is zero.Therefore, the following equation holds ##EQU6##

In the equation (5), if the base-emitter voltages VBE of the transistorsare negligible when compared with the input voltages V1 to V3, thefollowing equation holds ##EQU7##

In the above equation, the resistors R1 to R3 are all equal to eachother (R1=R2=R3=R). The collector current I1 of the transistor Q1 isobtained as the sum of the current I01 of the current source and thecurrent flowing through the resistor R1. The collector currents I2 andI3 are also obtained in a similar manner. ##EQU8##

When the equations (7-1) to (7-3) are compared with the equations (2-1)to (2-3) defining the output currents in the first embodiment with theΔ-connection resistor arrangement, the former is different from thelatter only in the proportional coefficient (mutual conductance).Therefore, the equations (7-1) to (7-3) are equivalent to those (2-1) to(2-3), respectively. From the equations (7-1) to (7-3), differencesbetween the two currents are given by ##EQU9##

The difference of the equations (8-1) to (8-3) from the equations (3-1)to (3-3) is only in the coefficients in the voltage terms.

A two-phase prior voltage-current converting circuit shown in FIG. 3will be described to show correctness of the operating principle of thepresent invention. The circuit shown in FIG. 3 may be considered as atwo-phase voltage-current converting circuit using the Y-connectionresistor arrangement. Also in this circuit, the sum of the currentsflowing into a mid-point is zero. Therefore, the following equationhold. ##EQU10##

Where VBE(Q1)=VBE(Q2) and R1=R2=R. From the equations (9) and (6), apotential VS at a mid-point in the Y-connection resistors in the case ofa N-phase voltage-current converting circuit is given by ##EQU11##where, R1=R2= . . . =RN=R.

In FIG. 3, the collector currents I1 and I2 of the transistors Q1 and Q2are given ##EQU12##

From the equations (11-1) and (11-2), and (7-1) to (7-3), the collectorcurrents II of the I-th transistor QI in the N-phase voltage-currentconverting circuit are given by ##EQU13##

In FIG. 3, a difference between the two collector currents is given by##EQU14##

In this case, the currents I01 and I02 of the current sources areassumed to be equal to each other. The equation (13) is equivalent tothe equations (8-1) to (8-3) in the three-phase voltage-currentconverting circuit. Thus, the operation principle of the presentinvention is also applicable for the known two-phase circuit.Consquently, the operation principle of the present invention iscorrect. We can obtain the collector current II and I(I+1) of the I-thand (I+1)th transistors QI and Q(I+1) in the N-phase converter, from theequation (12). ##EQU15##

From the equations (14-1) and (14-2), a difference between the twooutput currents in the case of the N-phase converter is generallyexpressed as ##EQU16##

The equation (15) is equivalent to the equations (8-1) to (8-3) and (13)in the three-phase and two-phase converters.

A third embodiment of the invention shown in FIG. 4 is thevoltage-current converting circuit arrangement with the Y-connectionresistors illustrated in FIG. 2 modified into the N-phasevoltage-current converting circuit. In the third embodiment, themid-point voltage VS is expressed by ##EQU17##

If ##EQU18## the equation (16) can be rewritten into ##EQU19##

The equation (17) is equal to the equation (10). This indicates that thethird embodiment also operates as the voltage-current convertingcircuit.

FIG. 5 illustrates an N-phase voltage-current converting circuit of theΔ-connection type according to a fourth embodiment of the presentinvention. The present embodiment is a modification of the firstembodiment shown in FIG. 1. As shown in FIG. 5, the emitter of atransistor QI (I: 1 to N) is connected to the emiter of a transistor QJ(J: I+1 to N) through a resistor RJI.

If the base-emitter voltage VBE of the transistors are equal to eachother, the collector currents II and I(I+1) of the I-th and (I+1)thtransistors are ##EQU20##

In the resistors RIJ are equal to one another and denoted as R, theequations (18-1) and (18-2) are expressed by ##EQU21##

A difference between the two currents is ##EQU22##

The equation (20) is equivalent to the equations (3-1) to (3-3) in thecase of the three-phase converter. The present embodiment also serves asthe voltage-current converting circuit.

The above-mentioned embodiments have been described on the assumptionthat the base-emitter voltages of the transistors are equal to eachother. An embodiment of the invention for compensating for an error dueto the ΔVBE will now be described. FIG. 6 shows a fifth embodiment whichis an improvement of the first embodiment of the three-phaseΔ-connection type shown in FIG. 1. In the present embodiment,operational amplifiers A1 to A3 are provided at the prestage of thetransistors Q1 to Q3, respectively. Three-phase input voltages V1 to V3are coupled with noninverting input terminals of the operationalamplifiers A1 to A3, respectively. The output signals from theoperational amplifiers A1 to A3 are respectively supplied to the basesof the transistors Q1 to Q3. The emitters of the transistors Q1 to Q3are coupled with the inverting input terminals of the operationalamplifiers A1 to A3, respectively.

With such an arrangement, the operational amplifiers A1 to A3 operate asvoltage followers because of their negative feedback action. The inputvoltages V1 to V3 are exactly transferred to the connection points ofthe resistors R12, R23 and R31, respectively. The errors due to thebase-emitter voltages VBE of the transistors are not contained in thecollector currents I1 to I3.

FIG. 7 shows a sixth embodiment of the three-phase Y-connection type andis a modification of the second embodiment of FIG. 2

FIG. 8 shows a seventh embodiment illustrating an actual arrangement ofa voltage-current converting circuit of the three-phase Δ-connectiontype. The present embodiment, like the fifth embodiment shown in FIG. 6,compensates for the error due to the base-emitter voltage VBE. Thepresent embodiment employs voltage follower circuits VF1 to VF3, inplace of the operational amplifiers A1 to A3 of the fifth embodiment. Inthe voltage follower circuits VF1 to VF3 and the converting circuit, thetransistors Q1 to Q3 and the current sources I01 to I03 are shared. Thevoltage follower circuits VF1 to VF3 directly output a voltage inputtedthereto. Input voltages V1 to V3 applied to the input terminals of theVF1 to VF3 are transmitted to the emitters of output transistors Q1 toQ3, respectively. Therefore, voltages at the connection points of theΔ-connection resistors R12, R23 and R31 are equal to the input voltages,respectively, thereby effecting an accurate voltage-current conversion.The output currents I1 to I3 converted are taken out from the collectorsof transistors Q1' to Q3' respectively connected in series to the outputtransistors Q1 to Q3. The present embodiment needs nine transistors forconstituting one voltage follower and therefore needs a total of 27transistors. The present embodiment is not preferable for an integratedfabrication because the number of elements is large. An embodiment ofthe invention which has successfully reduced the number of elements willbe described.

An eighth embodiment of the invention shown in FIG. 9 is different fromthe seventh embodiment in the arrangement of the voltage followercircuit. As shown, transistors Q11, Q12 and Q13 are connected in seriesbetween a current source I01 and an output terminal O1. Transistors Q21,Q22 and Q23 are connected in series between a current source I02 and anoutput terminal O2. Transistors Q31, Q32 and Q33 are connected in seriesbetween a current source I03 and an output terminal O3. Resistors R12,R23 and R31 are each between the emitters of the two transistors. Inputvoltages V2, V3 and V1 are applied to the bases of the transistors Q13,Q23 and Q33, respectively. The bases of the transistors Q11, Q21 and Q31are connected to the collectors of the transistors Q21, Q31 and Q11,respectively. The bases of the transistors Q12, Q22 and Q32 areconnected to the collectors of the transistors Q22, Q32 and Q12,respectively. If the emitter areas of these transistors are equal toeach other in the transistor circuit, a voltage between the base of thetransistor Q13 and the emitter of the transistor Q11 is equal to avoltage between the base of the transistor Q23 and the emitter of thetransistor Q21 and a voltage between the base of the transistor Q33 andthe emitter of the transistor Q31. When the emitter areas of thetransistors are different from one another, these voltages depend on theemitter areas of the transistors.

The operation of the embodiment thus arranged will be described. Theemitter voltages of the transistors Q11, Q21 and Q31, denoted as V1',V2' and V3' respectively, are given by the following equations.

    V1'=V1-{VBE(33)+VBE(22)+VBE(11)}                           (21-1)

    V2'=V2-{VBE(13)+VBE(32)+VBE(21)}                           (21-2)

    V3'=V3-{VBE(23)+VBE(12)+VBE(31)}                           (21-3)

In the circuit under discussion, the base voltages of the transistorsQ13, Q23 and Q33 are selected to be equal to each other. Thesetransistors are biased so as to operate in an active region, withapplication of a proper voltage to the collectors thereof. Upon biasingof these transistors, the transistors Q11 to Q13, Q21 to Q23 and Q31 toQ33 all operate in an active region. If the common emitter currentamplification factor of each transistor is satisfactorily large, thebase current of each transistor is negligible. If the transistors Q11 toQ13 have equal emitter areas, the collector currents of the transistorsQ11 to Q13 are all I1. Therefore, the base-emitter voltages of thesethree transistors are equal to each other

    VBE(11)=VBE(12)=VBE(13)=VBE1                               (22-1)

The same thing is true for the transistors Q21 to Q23 and Q31 to Q33,and we have

    VBE(21)=VBE(22)=VBE(23)=VBE2                               (22-2)

    VBE(31)=VBE(32)=VBE(33)=VBE3                               (22-3)

Substituting the equations (22-1) to (22-3) into the equations (21-1) to(21-3), we have

    V1'=V1-3VBE                                                (23-1)

    V2'=V2-3VBE                                                (23-2)

    V3'=V3-3VBE                                                (b 23-3)

where 3VBE=VBE1+VBE2+VBE3.

V1' to V3' are voltages at the connection points of the Δ-connectionresistors, and these are shifted by 3VBE from the input voltages V1 toV3. The shift amounts of the voltages are equal to each other. Thisindicates that the currents flowing through the resistors R12, R23 andR31 are the same as those when the input voltages V1 to V3 are directlyapplied to the resistors R12, R23 and R31. Therefore, the outputcurrents I1 to I3 are respectively expressed by the equations (2-1) to(2-3) and depend on the input voltages V1 to V3. According to the eighthembodiment, the converting accuracy can be improved without increasingthe number of elements. Thus, the voltage-current converting circuitprovided according to the eighth embodiment is suitable for theintegrated fabrication.

Description will be given about the ranges of the input voltages V2 andV3 with respect to the input voltage V1 as an operating condition of theeighth embodiment. A circuit arrangement useful in the description tofollow is illustrated in FIG. 10. A base of the transistor Q33 isgrounded and base voltages of the transistors Q13 and Q23 are ΔV1 andΔV2, respectively. The collector-emitter voltages of the transistorsQ32, Q22, Q12 and Q11 are -ΔV1+VBE, ΔV2+VBE, ΔV1-ΔV2+VBE, ΔV1-ΔV2+VBE,-ΔV1+VBE and ΔV2+VBE, respectively. For operating this circuit, thecollector-emitter voltages of the transistors must be positive. Hence,the following conditions hold

    ΔV1<VBE                                              (24-1)

    ΔV2>-VBE                                             (24-2)

    ΔV1-ΔV2+VBE>0                                  (24-3)

The conditions of ΔV1 and ΔV2, illustrated in FIG. 11, are derived fromthe equations (24-1) to (24-3). In the graph, a hatched area indicatesan operating area.

The result of a computer simulation of the eighth embodiment will begiven. The circuit of FIG. 9 is used for the simulation. The inputvoltages are three-phase voltages V1 to V3 with 120° phase difference,as shown in FIGS. 12A to 12C. The amplitude of each input voltage is 0.1V. The circuit constants are: I01=I02=I03=100 μA, R12=R23=R31=10 KΩ. Thecollector voltage of each of the transistors Q13, Q23 and Q33 is 1 V.The output currents I1 to I3 can be theoretically obtained from theequations (2-1) to (2-3). These theoretical values are shown in FIGS.13A to 13C. The results of the simulation by a computer every 2 μsec areshown in FIGS. 14A to 14C. These results show that the simulation valueshave a high correspondence to the theoretical values. The results of thesimulation show that a maximum error is a mere -3%. The error is causedwhen the currents converted from the input voltages by the resistorsR12, R23 and R31 pass through three transistors till the outputterminals O1, O2 and O3.

FIG. 15 shows a circuit arrangement of a ninth embodiment which is amodification of the eighth embodiment into an N-phase voltage-currentconverting circuit. The present embodiment is designed so as tocompensate for the error due to the VBE in the voltage-currentconverting circuit of the N-phase Y-connection type shown in FIG. 4. InFIG. 15, if the Y-connection resistor arrangement is substituted by aΔ-connection resistor arrangement, the FIG. 15 circuit is a modificationof the FIG. 5 embodiment.

What is claimed is:
 1. A voltage-current converting circuitcomprising:at least three input transistors, the bases of saidtransistors receiving a multiphase voltage, the collectors of saidtransistors being connected to respective output terminals, and theemitters of said transistors being respectively connected to their ownbases through voltage follower circuits; voltage-current convertingmeans including a plurality of Δ-connection resistors, each of saidresistors being connected between the emitters of two adjacent inputtransistors; and current sources of the same number as said inputtransistors and connected to the emitters of said input transistors,respectively.
 2. A voltage-current converting circuit according to claim1 in which said voltage follower circuits each comprise an operationalamplifier which has a multiphase input applied to its noninverting inputterminal, and which has its output terminal and inverting input terminalconnected respectively to the base and the emitter of the correspondinginput transistor.
 3. A voltage-current converting circuit comprising:atleast three input transistors applied at the bases with a multiphasevoltage and connected at the collectors to respective output terminals;current sources of the same number as said input transistors andconnected to the emitters of said input transistors; and voltage-currentconverting means including a plurality of Δ-connection resistors, eachof said resistors being connected between the emitters of two adjacentinput transistors, the emitters of said input transistors beingrespectively connected through the collector-emitter paths of across-coupled group of transistors to ends of the resistors, the base ofone transistor being connected to the collector of another transistor insaid cross-coupled group of transistors.
 4. A voltage-current convertingcircuit comprising:at least three input transistors, the bases of saidtransistors receiving a multiphase voltage, the collectors of saidtransistors being connected to respective output terminals, and theemitters of said transistors being respectively connected to their ownbases through voltage follower circuits; voltage current convertingmeans including a plurality of Y-connection resistors, each of saidresistors having one end connected to a respective one of the emittersof said input transistors, with the other end of said resistors beingconnected together; and current sources of the same number as said inputtransistors and connected to the emitters of said input transistors,respectively.
 5. A voltage-current converting circuit according to claim4 in which said voltage follower circuits each comprise an operationalamplifier which has a multiphase input applied to its noninverting inputterminal, and which has its output terminal and inverting input terminalconnected respectively to the base and the emitter of the correspondinginput transistor.
 6. A voltage-current converting circuit comprising:atleast three input transistors applied at the bases with a multiphasevoltage and connected at the collectors to respective output terminals;current sources of the same number as said input transistors andconnected to the emitters of said input transistors; and voltage-currentconverting means including a plurality of Y-connection resistors, eachof said resistors having one end connected to a respective one of theemitters of said input transistors with the other ends of said resistorbeing connected together, the emitters of said input transistors beingrespectively connected through the collector-emitter paths of across-coupled group of transistors to ends of the resistors, and thebase of one transistor being connected to the collector of anothertransistor in the cross-coupled group of transistors.